This invention relates to a process for the protection of rolled sheets, as a rule previously galvanized, particularly suited for use in the motor-car industry and moreover, relates to the product obtained by such a process. It is known to use electrolytic treatments based on Cr-CrOx layers on bars (unshielded) steel. As shown by U.K. Pat. No. 1,247,881, U.S. Pat. No. 3,642,587, and French Pat. No. 2,003,981, such electrolytic treatments are resorted to essentially with a view to substituting, in some cases, the white latten usually applied to metal containers and to constitute a pretreatment of a bare steel side before its hot dip galvanizing, the purpose being to prevent zinc from adhering to one of the sides, and to obtain a hot dip one side product.
French Pat. No. 2,053,038 on the other hand relates to a single-stage treatment successive to the galvanizing operation, which does not cause a multi-layer plating, but a Cr+CrOx mixture, wherein CrOx is predominant. Also the maximum deposit amount is equal to 0.650 g/m.sup.2, which, according to experience, is suuboptimal for corrosion protection. Moreover, the treatment according to said French Pat. No. 2,053,038 has inconveniences so far as its industrial implementation is concerned, and its plating is unbalanced in the direction of high CrOx contents, whichh leads to a critical dissolution of the plating in either acid baths (phosphating) or significantly alkaline baths prepainting washings). Consequently there are two unacceptable effects, particularly in the motor-car industry; a great variance in corrosion-resistance tests, and contamination of phosphating baths.
Based on German patent application No. 2,114,333, there is also known a one- or two-stage process for the multi-layer treatment of galvanized or zinc-alloy plated products, according to which a Cr-CrOx plating is applied on the zinc layer, the said plating having a protective function on the galvanized product.
The process according to the above patent application essentially derives from an experiment in plating galvanized steel wires and cables. However, the possibility of extending it to flat products is also mentioned. Perhaps on account of this lack of application experience to flat rolled sheets, the process conditions specified therein and verified industrially proved unsuited to obtain a Cr-CrOx plating suitable for successive phosphating and painting processes without functional and ecologic inconveniences.